Many semiconductor imaging sensors today are front side illuminated. That is, they include imaging arrays that are fabricated on the front side of a semiconductor wafer, where light is received at the imaging array from the same front side. However, front side illuminated imaging sensors have many drawbacks, one of which is a limited fill factor.
Backside illuminated imaging sensors are an alternative to front side illuminated imaging sensors that address the fill factor problems associated with front side illumination. Backside illuminated imaging sensors include imaging arrays that are fabricated on the front surface of the semiconductor wafer, but receive light through a back surface of the wafer. However, to detect visible light from the backside, the wafer must be extremely thin. Micro-lenses may be included on the back surface of the wafer in order to improve the sensitivity of the backside illuminated sensor. The thickness of the wafer may be optimized in order to improve the spectral performance and reduce crosstalk. That is, as the final semiconductor wafer thickness is increased, light can be more effectively collected by the wafer. This is particularly true for red light, which penetrates deeper into silicon before being absorbed. At the same time, blue light, which is absorbed near the surface, may generate photoelectrons that recombine at the back surface, return to the back surface or are collected at a neighboring pixel. Photoelectrons that recombine at the back surface of the silicon may lead to reduced sensitivity, while photoelectrons that are collected at a neighboring pixel may lead to crosstalk of the imaging sensor. As the final semiconductor wafer thickness is decreased, more of the red light passes completely through the collection region without being absorbed and thereby further reduces sensitivity. Thus, a need exists for a backside illuminated device with improved sensitivity that reduces crosstalk.